Methods, systems and apparatus for paced breathing

ABSTRACT

Methods, systems and/or apparatus for slowing a patient&#39;s breathing by using positive pressure therapy. In certain embodiments, a current interim breathing rate target is set, and periodically the magnitude of a variable pressure waveform that is scaled to the current interim breathing rate target is increased if the patient&#39;s breathing rate is greater than the interim breathing rate target in order to lengthen the patient&#39;s breath duration. The magnitude of the pressure increase may be a function of the difference between the interim breathing rate target and the patient&#39;s breathing rate. The interim breathing rate target may be periodically reduced in response to the patient&#39;s breathing rate slowing down toward the current interim breathing rate target. The variable pressure waveform cycles from an inhalation phase to an exhalation phase when the patient airflow decreases to a cycle threshold, the cycle threshold being a function of flow versus time within a breath and generally increasing with time. Different interim breathing rate targets have different cycle threshold functions, and the cycle threshold functions allow easier cycling as the interim breathing rate targets decrease. Similarly, the variable pressure waveform triggers from an exhalation phase to an inhalation phase when the patient airflow increases to a trigger threshold, the trigger threshold being a function of flow versus time within a breath and generally decreasing with time. Different interim breathing rate targets have different trigger threshold functions, and the trigger threshold functions allow easier triggering as the interim breathing rate targets decrease.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.12/422,411, filed Apr. 13, 2009, which claims the benefit of U.S.Provisional Application No. 61/045,161 filed on Apr. 15, 2008. Bothrelated applications, in their entirety, are each incorporated herein byreference.

FIELD OF THE INVENTION

The disclosure relates to methods, systems and apparatus for pacedbreathing.

BACKGROUND OF THE INVENTION

Hypertension and cardiac failure are both diseases that cannot always becontrolled with current medications. With hypertension, any decrease inblood pressure (particularly pulse pressure) is beneficial. Hypertensiveand cardiac failure patients possess heightened sympathetic tone (higherbasal activity) and chemoreflex (hypoxic, relating to deficiencies ofoxygen) response (a reflex initiated by the stimulation ofchemoreceptors, which are specialized cells for detecting chemicalsubstances and relaying that information centrally in the nervoussystem). Other conditions are also associated with heightenedsympathetic tone, which can impart extra load on the heart and otherorgans. Arterial stiffness, directly related to sympathetic activationand blood pressure, is attracting abundant clinical attention presently,and methods which directly reduce it are now a goal in themselves.

Slow breathing improves arterial baroreflex sensitivity and decreasesblood pressure in essential hypertension, which continuous positiveairway pressure (CPAP) may assist by avoiding nightly repetitivedesaturation and arousal. For example, in certain situations slowedbreathing, at 6 breaths per minute, has been shown to be beneficial.Slowed breath rate can independently improve sympathovagal balance, ofpotential benefit to the general population interested in maximizingcardiovascular health or minimizing stress. (“Sympathovagal balance”refers to the autonomic state resulting from both sympathetic andparasympathetic influences; the autonomic nervous system directs allactivities of the body that occur without a person's conscious control,such as breathing and food digestion. Typically, it has two parts: thesympathetic division, which is most active in times of stress, and theparasympathetic division, which controls maintenance activities andhelps conserve the body's energy.)

Severe chronic obstructive pulmonary disease (COPD, a term referring totwo lung diseases, chronic bronchitis and emphysema) is a condition ofairway flow limitation, associated with hypercapnia (a condition inwhich there is too much CO₂ in the blood) and hypoxaemia (a condition inwhich there is too little O₂ in the blood), increased respiratory muscleloading, diminished exercise capacity, elevated respiratory rate, etc.Noninvasive positive pressure ventilation (NPPV, delivery of ventilatorysupport using a mechanical ventilator connected to a mask or mouthpiece)sessions promoting deep and slowed breathing have been shown to bebeneficial, possibly due to a lowered effective respiratory impedance,and a deep slow pattern of breathing has been maintained by patientsbetween sessions.

Therapies exist to help train a patient to consciously breathe slowlyusing acoustic feedback/training. See, for example, the system known asRESPERATE. See also PCT Publication Number WO 2008/021222, whichdiscloses use of a CPAP machine to reduce a patient's breathing rate.

Thus a paced-breathing systems, methods and/or apparatus that achievessustained target breath rate in a comfortable and/or tolerated fashionare needed. The therapy may be delivered as daytime sessions ofprescribed duration and/or as a nocturnal therapy. The therapy goal isto modify breath rate by delivering mechanical ventilation optimized toachieve a rate target, but sympathetic to the response of the patientsuch that the therapy is well tolerated. In certain embodiments, thegoal is to lower rate to a value optimized to suit the patient and/orthe pathology. In certain embodiments, the goal may be to reach anoptimal rate, possibly higher than the patient's spontaneous rate.

SUMMARY OF THE INVENTION

Certain embodiments of the present disclosure are directed to slowing apatient's breathing during sleep and/or during awake sessions by usingpositive pressure therapy.

Certain embodiments of the disclosure are directed to providing for apatient improved cardiovascular health without and/or minimal sideeffects.

Certain embodiments of the disclosure are directed to applying therapyduring sleep (rather than during specific periods of the day) so that alonger treatment session at greater personal convenience is achieved.

Certain embodiments of the disclosure are directed to achieving theforegoing while resolving at the same time certain patient sleepdisordered breathing (SDB).

In certain embodiments, following a settling period during sleep onset,during which breathing is largely spontaneously triggered, a variablepressure waveform is applied to the patient's airway. An interimbreathing rate target may be set, and periodically the magnitude of thepressure waveform is changed (for example increased) when the patient'sbreathing rate is greater than the interim breathing rate target inorder to lengthen the patient's breath duration. Periodically, inresponse to the patient's breathing rate slowing down toward the currentinterim breathing rate target, the interim breathing rate target isreduced. The reduction may follow a predetermined path, but thereduction may be paused if, for example, the patient's breathing rate isexcessively high for the current interim breathing rate target.

In certain embodiments, the variable pressure waveform cycles from aninhalation phase to an exhalation phase when the patient airflowdecreases to a cycle threshold, the cycle threshold being a function offlow versus time within a breath and generally increasing with time.Different interim breathing rate targets have different cycle thresholdfunctions, and the cycle threshold functions allow easier cycling as theinterim breathing rate targets decrease. Similarly, the variablepressure waveform triggers from an exhalation phase to an inhalationphase when the patient airflow increases to a trigger threshold, thetrigger threshold being a function of flow versus time within a breathand generally decreasing with time. Different interim breathing ratetargets have different trigger threshold functions, and the triggerthreshold functions allow easier triggering as the interim breathingrate targets decrease.

In certain embodiments, pressure adjustments are not always upward. Themagnitude of the pressure waveform is decreased if the patient'sbreathing rate is less than the interim breathing rate target. Ingeneral, in certain embodiments, the magnitude of a pressure increase ordecrease is a function of the difference between the interim breathingrate target and the patient's breathing rate. The duration of thepressure waveform may be adjusted in accordance with the current interimbreathing rate target.

Another way of looking at the certain embodiments of the presentdisclosure is to view it as the use of a ventilator coupled to apatient's airway, where an interim breathing rate target is set, thepressure support supplied to the patient's airway from the ventilatorduring patient breaths (not necessarily all breaths) is increased if thepatient's breathing rate is greater than the interim breathing ratetarget in order to lengthen the patient's breath duration, and theinterim breathing rate target is reduced in response to the patient'sbreathing rate slowing down toward the interim breathing rate target.The pressure-support increase and interim breathing rate targetreduction steps are interrupted if the patient exhibits opposition tobreath duration lengthening, but otherwise the air increase and interimbreathing rate target reduction steps are controlled to take place overa period of minutes to hours in the absence of patient opposition.

Since brief (15 minute) sessions of slow breathing while awake offer abeneficial reduction in blood pressure or sympathetic activation, thesame principle (slowed breathing) successfully achieved during sleep viaa positive pressure therapy offers greater benefit (longer therapy time)while simultaneously avoiding obstructive sleep apnea (OSA) andrespiratory effort related arousal (RERA) activity, and it is more timeefficient for the patient. Over time, the patient's spontaneous rate isreduced as the chemoreflex normalizes, but in advance of that occurring,the patient enjoys hours at the slower (therapeutic) rate during sleep,compared with minutes with the conscious method. For the case of COPD,whether the paced breathing system is used during daytime sessions ornocturnally while asleep, by algorithmically lowering rate, compared totraditional NPPV, the paced breathing system offers clinical convenience(reduced, little, or no supervision necessary) and it optimizesoutcomes. In certain embodiments, the patient may also achieve thesebenefits if awake and/or asleep.

In certain embodiments, the goal is to entrain the patient to an idealbreath rate, e.g., lowered to 6 breaths per minute or as low as thepatient will tolerate. In certain embodiments, the goal is to entrainthe patient to a breath rate, e.g., lowered to approximately 12, 11, 10,9, 8, 7, 6, 5, 4, or 3 breaths per minute over a desired time period.This time period may be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% ofthe time that the patients is using the method, system, and/orapparatus. This low rate is partially achieved through maximizing, orsubstantially maximizing, inspiratory duration through elevated tidalvolume, but much of the rate retardation is achieved by sustainingexpiration and promoting an end-expiratory pause.

In certain embodiments, a factor is slow pressurization andde-pressurization. Fast pressurization (inspiratory positive airwaypressure—IPAP) is known to hasten inspiration, to be avoided, whiletotal occlusion of the patient airway during inspiration tends tosustain inspiratory effort. A slow pressurization will slow downinspiration, or at least not hasten it. Similarly, once cycling occurs,the de-pressurization is controlled and tapered such that it reaches thetarget EPAP (end positive airway pressure) just prior to the targetinspiratory time, again to slow breathing. (For patients withobstructive airway disease, suffering slow lung inflation/deflationalready, and also suffering dyspnoea, a slow pressurization and recoilmay be in certain situations disadvantageous, and a squarer waveform maybe selected based on preference or application.) Once the patient'sbreathing rate is reduced to an interim target rate, the basic mechanismof certain embodiments is to increase the pressure slightly; for minutevolume (volume of air which can be inhaled or exhaled from a person'slungs in one minute) to remain the same, the patient tends to lengthenthe breath duration. The target rate is then reduced again, hopefullyfollowed by another lengthening of the patient's breath as he/shemaintains his/her minute volume.

In certain embodiments, the trigger scheme permits the patient tobreathe at his/her preferred spontaneous rate, but aims to encourage theoptimal, or substantially optimal, rate by having a trigger thresholdthat initially is insensitive, or substantially insensitive, and whichbecomes most sensitive, or more sensitive, only as the ideal, ordesired, breath period is approached. And for the interim target rate,following expiratory cycling there is an initial refractory period wheretriggering is insensitive, or substantially insensitive, followed byprogressive sensitization, slowly approaching a more typical triggerthreshold by the desired (for the current interim target rate)end-expiratory period (e.g., sensitivity progressively increasing from,for example, 20 L/min to 5-7 L/min). Similarly, the cycling scheme thatis employed encourages cycling at the target inspiratory durationthrough progressive change in cycle threshold from highly insensitive tohighly sensitive by the target inspiratory time. In certain aspects, thecycling scheme that is employed encourages cycling at the targetinspiratory duration through progressive change in cycle threshold frominsensitive to sensitive by the target inspiratory time.

Although not shown, in certain embodiments it is contemplated thatmonitoring of the patient can include at least one of, or combinationsthereof of respiratory parameters and customized pulse oximetry, andcapturing of instantaneous heart rate suitable for heart ratevariability (HRV) or similar analysis. Also, daytime sessions can beaugmented via biofeedback for inspiratory/expiratory timing, e.g.,acoustic methods such as RESPERATE mentioned herein or discerniblepneumatic fluctuations communicated via the patient's CPAP mask, visualcues, or combinations thereof.

In certain embodiments, the disclosed methods, systems, and/or apparatusoperate so as to synchronize to the patient's breathing rate rather thanto operate in accordance with the interim breathing rate target. Incertain aspects, the disclosed methods, systems, and/or apparatus arenot synchronized to an interim breathing rate target in certain aspects,the ventilator synchronizes to the patient's breathing, by detecting achange in the patient's breathing phase. An attempt is not made to lowerthe patient's breathing rate by slowing down the ventilator in the hopethat the patient will ‘synchronize’ to the ventilator. The ventilatorinstead increases the pressure support to the patient in the hope thatthe patient will slow down his breathing rate, while the ventilatorremaining synchronized to the patient's breathing rate.

In certain embodiments, even though the ventilator operation issynchronized to phase changes in patient breathing, the pressurewaveform template changes its scale to that of a new, longer interimtarget breath period. In other words, the ventilator supplies a pressurewaveform whose duration corresponds with a longer breath period, eventhough a patient phase change will abort a ventilator phase in progress.This conditions the patient to breathe more slowly without attempting toforce a rate change; the ventilator attempts to lead the patient to aslower rate.

In certain embodiments, the pressure is continuously adjusted. Not onlyis a continuously variable pressure template used for each breathingcycle, but the amplitude of the waveform is adjusted from cycle tocycle. In certain embodiments, the pressure may be continuously,substantially continuously and/or capable of being continuouslyadjusted. In certain aspects, the pressure template used for breathingcycles may be continuously, substantially continuously and/or capable ofbeing continuously adjusted. In certain aspects the amplitude of thewaveform may be continuously, substantially continuously and/or capableof being continuously adjusted from cycle to cycle. In certain aspects,the pressure, the pressure template, the wave form or combinationsthereof may be continuously, substantially continuously and/or capableof being continuously adjusted.

In certain embodiments, the target rate is changed not only as afunction of recent breathing periods of the patient, but also inaccordance with a predetermined gradient that may require, hours days oreven weeks before the desired rate is achieved.

In certain embodiments, each phase length is independently operatedupon. In certain embodiments, a substantial number of the phase lengthsare independently operated upon.

In certain embodiments, the thresholds for determining whether a changein breathing phase should be effected continuously change not onlywithin a breath, but also with the current breathing rate. In certainembodiments, the thresholds for determining whether a change inbreathing phase should be effected continuously, substantiallycontinuously or is capable of being continuously changed not only withina breath, but also with the current breathing rate.

Certain embodiments are to methods and/or systems for slowing apatient's breathing during sleep by using positive pressure therapycomprising the steps of applying a variable pressure waveform to thepatient's airway, setting a current interim breathing rate target,periodically increasing the magnitude of the pressure waveform when thepatient's breathing rate is greater than the interim breathing ratetarget in order to lengthen the patient's breath duration, andperiodically reducing the interim breathing rate target in response tothe patient's breathing rate slowing down toward the current interimbreathing rate target.

Certain embodiments are to methods and/or systems or slowing a patient'sbreathing during sleep by using positive pressure therapy comprising thesteps of setting an interim breathing rate target, increasing themagnitude of a variable pressure waveform that is scaled to the interimbreathing rate target if the patient's breathing rate is greater thanthe interim breathing rate target in order to lengthen the patient'sbreath duration, the magnitude of the pressure increase being a functionof the difference between the interim breathing rate target and thepatient's breathing rate, and reducing the interim breathing rate targetin response to the patient's breathing rate slowing down toward or belowthe interim breathing rate target.

Certain embodiments are to methods and/or systems for slowing apatient's breathing during sleep by using a ventilator coupled to apatient's airway comprising the steps of setting an interim breathingrate target, increasing the air supplied to the patient's airway fromthe ventilator during patient breaths if the patient's breathing rate isgreater than the interim breathing rate target in order to lengthen thepatient's breath duration, reducing the interim breathing rate target inresponse to the patient's breathing rate slowing down toward the interimbreathing rate target, and interrupting the air increase and interimbreathing rate target reduction steps if the patient exhibits oppositionto breath duration lengthening, the air increase and interim breathingrate target reduction steps being controlled to take place over a periodof minutes to hours in the absence of patient opposition.

Certain embodiments are to devices for slowing a patient's breathingduring sleep by using positive pressure therapy comprising a blower forapplying a variable pressure waveform to the patient's airway, at leastone sensor for detecting the breathing rate of the patient, and acontroller for setting a current interim breathing rate target, causingthe blower to periodically increase the magnitude of the pressurewaveform when the patient's breathing rate is greater than the interimbreathing rate target in order to lengthen the patient's breathduration, and periodically reducing the interim breathing rate target inresponse to detection of the patient's breathing rate slowing downtoward the current interim breathing rate target.

Certain embodiments are to devices for slowing a patient's breathingduring sleep by using positive pressure therapy comprising a blower forapplying a variable pressure waveform to the patient's airway, at leastone sensor for detecting the breathing rate of the patient, and acontroller for setting an interim breathing rate target, causing theblower to increase the magnitude of a variable pressure waveform that isscaled to the interim breathing rate target if the patient's breathingrate is greater than the interim breathing rate target in order tolengthen the patient's breath duration, the magnitude of the pressureincrease being a function of the difference between the interimbreathing rate target and the patient's breathing rate, and reducing theinterim breathing rate target in response to the patient's breathingrate slowing down toward the interim breathing rate target.

Certain embodiments are to devices for slowing a patient's breathingduring sleep comprising a blower coupled to a patient's airway, at leastone sensor for monitoring the breathing of the patient, and a controllerfor setting an interim breathing rate target, causing the air suppliedto the patient's airway from the blower during patient breaths toincrease if the patient's breathing rate is greater than the interimbreathing rate target in order to lengthen the patient's breathduration, reducing the interim breathing rate target in response to thepatient's breathing rate slowing down toward the interim breathing ratetarget, and interrupting air increase and interim breathing rate targetreduction if the patient exhibits opposition to breath durationlengthening, the controller causing the air increase and interimbreathing rate target reduction to take place over a period of minutesto hours in the absence of patient opposition.

In certain embodiments, the variable pressure waveform cycles from aninhalation phase to an exhalation phase when the patient airflowdecreases to a cycle threshold, the cycle threshold being a function offlow versus time within a breath and generally increasing with time.

In certain embodiments, the different interim breathing rate targetshave different cycle threshold functions, and the cycle thresholdfunctions allow easier cycling as the interim breathing rate targetsdecrease.

In certain embodiments, the variable pressure waveform triggers from anexhalation phase to an inhalation phase when the patient airflowincreases to a trigger threshold, the trigger threshold being a functionof flow versus time within a breath and generally decreasing with time.

In certain embodiments, the different interim breathing rate targetshave different trigger threshold functions, and the trigger thresholdfunctions allow easier triggering as the interim breathing rate targetsdecrease.

In certain embodiments, the magnitude of the pressure waveform isdecreased if the patient's breathing rate is less than the interimbreathing rate target.

In certain embodiments, the magnitude of a pressure increase or decreaseis a function of the difference between the interim breathing ratetarget and the patient's breathing rate.

In certain embodiments, the interim breathing rate target is reducedalong a predetermined path, but the reduction is paused if the patient'sbreathing rate is excessively high for the current interim breathingrate target.

In certain embodiments, the duration of the pressure waveform isadjusted in accordance with the current interim breathing rate target.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain illustrative embodiments will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows apparatus according to certain embodiments;

FIG. 2A is a generalized block representation of a closed-loop(feedback) control system, in accordance with certain embodiments;

FIG. 2B, which is based on FIG. 2A, depicts a closed-loop controller forpaced breathing, with the ultimate aim of maintaining a low respiratoryrate optimized for a condition of interest, within limits achievable fora given patient, in accordance with certain embodiments;

FIGS. 3A and 3B are general (within-session) time-courses for breathpacing therapy towards the optimal breath rate, showing respectivelybreath rate and pressure support, in accordance with certainembodiments;

FIG. 4 is similarly a (within-session) time-course for breath pacingtherapy, responding to patient opposition, in accordance with certainembodiments;

FIG. 5 is a conditioning time-course for breath pacing therapy, showinghow the optimal breath rate may be achieved over many days, inaccordance with certain embodiments;

FIGS. 6A and 6B depict cycling behaviour, aiming to progressively extendthe inspiratory phase, early in the rate-lowering process where theinterim target inspiratory time (progressive Ti target) is considerableshorter than the optimal Ti, in accordance with certain embodiments;

FIGS. 7A and 7B depict cycling behaviour, aiming to progressively extendthe inspiratory phase, towards the end of the rate-lowering process,where the patient's spontaneous inspiratory phase is almost at theoptimal Ti, in accordance with certain embodiments;

FIGS. 8A and 8B depict triggering behaviour, aiming to progressivelyextend the expiratory phase, early in the rate-lowering process wherethe interim target expiratory time is considerably shorter than theoptimal target Te, in accordance with certain embodiments;

FIGS. 9A and 9B depict triggering behaviour, aiming to progressivelyextend the expiratory phase, towards the end of the rate-loweringprocess, where the patient's spontaneous expiratory phase is almost atthe optimal Te, in accordance with certain embodiments; and

FIG. 10 shows the flow control logic for the breath-pacing algorithm, inaccordance with certain embodiments.

DETAILED DESCRIPTION

FIG. 1 shows, by way of example, apparatus suitable for performingcertain disclosed embodiments. The apparatus includes an impeller 1connected to an electric motor 2 under the control of a servo-controller3 which is in turn under the control of a controller 4. In one form thecontroller 4 is a micro-processor based controller. The impeller 1 andmotor 2 form a blower. Air from the blower passes along a flexibleconduit 6 to a patient interface such as a nasal mask 5 with a vent 9.While a nasal mask is illustrated, certain disclosed embodiments may beused in conjunction with a nose-and-mouth mask, full face mask,endotracheal tube or other devices that perform the desired function. Anumber of switches 7 are connected to the controller. A number ofsensors are also connected to the controller, namely, sensors for flow10, pressure 11, snore 12, motor speed 13 and motor current 14. A set ofdisplays 8 connected to the controller 4 display information from thecontroller.

An interface 15 enables the controller 4 to communicate with an externaldevice such as a computer. With such a device, changes in the speed ofthe blower may be controlled to alternatively change the pressure in themask to implement ventilatory support. Optionally, the blower motorspeed may be held generally constant and pressure changes in the maskmay be implemented by controlling an opening of a servo-valve (notshown) that may variably divert/vent or deliver airflow to the mask.Those skilled in the art will recognize other devices for generatingventilatory support and delivering same to a patient.

The controller 4 or processor is configured and adapted to implementcertain of the methodologies described herein and may include integratedchips, a memory and/or other instruction or data storage media. Forexample, programmed instructions with the control methodology may becoded on integrated chips in the memory of the device or suchinstructions may be loaded as software. With such a controller, theapparatus can be used for many different pressure ventilation therapiessimply by adjusting the pressure delivery equation that is used to setthe speed of the blower or to manipulate the venting with the releasevalve. Those skilled in the art will also recognize that aspects of thecontroller may also be implemented by analog devices or other electricalcircuits.

The apparatus can further include a communication module, for example, awireless communication transceiver and/or a network card, forcommunication with other devices or computers such as hand-held displayand control devices. The apparatus optionally includes an oximeter inthe main blower housing. A sense tube may be connected to the mainhousing of the blower or the mask to allow the apparatus to sense oxygenconcentration and pressure levels in the mask. The apparatus may furtherinclude additional diagnosis units such as a pulse oximeter andrespiratory movement sensors. The unit may also include a set ofelectrodes for detecting cardiac rhythm.

It is understood that a combination of devices and/or computers linkedby available communications methods may be used to accomplish thedesired goals. For example, the apparatus can interface with a varietyof hand-held devices such as a Palm Pilot via wireless communication.With such a device, a physician may, for example, remotely monitor,analyze or record the status or data history of a patient or diagnosethe severity of the patient's condition using the device. Furthermore,the treatment program that is being run on the patient can be monitoredand changed remotely.

The generalized closed-loop servo control system of FIG. 2A operateswith a ‘target’ input, which is the desired value for some outputparameter of the servo system. A ‘controlled variable’ determines theoutput parameter and is often the output parameter itself. A feedbacktransducer 24 measures the controlled variable and an analysis andconditioning system 25 derives a signal that represents the controlledvariable in the same terms as the target input. Subtractor 20 generatesan error signal which controller 21 and perhaps one or more additionalcontrol elements such as final control element 22 shown in the drawingoperate on to derive a manipulated variable. That variable affects aplant process 23 to generate the controlled variable that is fed back.

FIG. 2B illustrates how the servo system of certain embodimentssatisfies the generalized closed-loop system of FIG. 2A. The controlledvariable is the patient's respiratory rate, and the target input is thedesired (interim) respiratory rate. The error signal, which representsthe difference between the actual and desired respiratory rates, is fedto the controller which derives a number of variables that are used inthe processing. Two of these are Target Ti and Target Te. Target Ti isan interim desired time interval for the inspiratory phase of a breath.It is not the final desired inspiratory time interval. It is only thedesired interval at the present time, on the way toward a longerinspiratory interval at the end of the overall process. The same appliesto Te, the interim desired expiratory time interval.

As the total breath interval lengthens (as Ti and Te lengthenindividually), the pressure support increases since more air must besupplied during each breath to compensate for fewer breaths overall.Thus there is a Target PS that is derived as well, an interim value thatrepresents the magnitude of the pressure waveform. The minimum pressureis fixed, so PS determines the maximum pressure.

The fourth and fifth variables derived by the controller based on thepresent error are Trigger sensitivity and Cycle sensitivity. The formerrepresents the value of patient air flow during expiration that willcause the machine to switch from an expiratory phase to an inspiratoryphase, and the latter represents the value of patient air flow duringinspiration that will cause the machine to switch from an inspiratoryphase to an expiratory phase. The five variables control the turbine orother mechanism used to supply pressure to a patient mask or otherpatient interface, which may be any commercial blower and mask used byCPAP patients, and the five variables are shown in the waveform thatrepresents the pressure profile at the output of the turbine. Thepressure applied rises to the Target PS level near the end of theinterim Target Ti. When the air flow is such that the Cycle sensitivitythreshold is reached, the machine switches to its expiratory phase. Thepressure then decreases toward the minimum level near the end of theTarget Te interval. When the air flow is such that the Triggersensitivity threshold is reached, the machine switches to itsinspiratory phase.

Referring back to the generalized block diagram of FIG. 2A, the plantprocess is the patient himself/herself. The input to the process is thepressure in the mask (indicated as the pressure profile). There are manyparameters that are affected in the control process, including, but notlimited to, respiratory flow and leak flow, but the output of interestis respiratory rate. The feedback transducer of FIG. 2A is in this caseflow sensor 28. The sensor estimates the respiratory flow, and from thisthe analysis and conditioning circuit 25 determines the current(interim) respiratory rate. This is the rate that is used to derive theerror. The individual functions of the servo system of FIG. 2B areperformed by standard commercial CPAP machines.

The waveform shown at the output of flow sensor 28 is the estimatedrespiratory flow. It is the current rate that is of interest and used inthe error calculation. The waveform also shows the value of Ttot, thetotal time duration of the current breath (the reciprocal of the currentrespiratory rate). The sum of Target Ti and Target Te, shown in thewaveform at the output of turbine 26, equals Ttot.

In certain embodiments, the pressure waveform is not two-valued as it isin other bi-level systems. A template is used for the pressure profile,as disclosed in many prior art patents. The turbine is controlled toprovide a pressure waveform that follows the template. The amplitude mayvary with PS, and the time may vary as the two target values change, butthe shape remains the same. It is as though the two axes of the waveformare shortened and lengthened, as the shape remains constant. However, itis not necessary that the shape remain constant, and even asquare-shaped or trapezoidal bi-level waveform can be used although itis generally recognized that in non-obstructed lungs, rapidpressurization such as with a square waveform tends to increase raterather than decrease it. Similarly, it is not necessary to have a fixedlower pressure (determined by the physician) and a variable upperpressure, as standard titrating mechanisms can be employed as well.

FIGS. 3A and 3B are general (within-session) time-courses for breathpacing therapy towards the optimal breath rate, showing respectivelybreath rate and pressure support. The initial portion depictsspontaneous breathing. When the device is first applied to the patient(event 1), a comfortable variable-level therapy regime is commenced(using the standard pressure profile template), with the machine ratedictated by the patient's spontaneous breathing (spontaneous mode). Theoperation is similar to standard VPAP therapy, with changes in machinephase following the patient's breathing. At the commencement of thepositive airway pressure (PAP) pacing algorithm (event 2), the therapyadopts an initial target breath rate adapted to the patient's own rate(solid dashed trace). After a prescribed settling time, the algorithmencourages a progressively slower rate, i.e., breath pacing (event 3).Through incremental adjustment of pressure support, and trigger/cyclethresholds, the target rate is incrementally slowed, the long-term goalbeing to achieve an optimal breath-rate (event 3 d), as set by theclinician. How variables are adjusted will be described herein. Theprogressive breath-rate target contour (faint dashed line) that isfollowed to achieve this optimal rate may be any shape (linear descentis shown), and is revised (events 3 a, 3 b, 3 c) along the way accordingto the patient's progress towards the target. The target rate is only an“interim” target rate—the servo control adjusts the pressure so as toslow down the breath rate so it meets the interim target, followingwhich a new, lower interim target comes into play. The result is astandard-type of servo control, with the input changing to conform to apredetermined strategy but at a speed that depends on the patient'sprogress.

It will be seen from FIG. 3A that when the patient breath rate (solidline) does not conform to the reducing target rate, i.e., when thepatient breath rate is too high, the target rate remains fixed until thepatient rate “catches up” (by slowing down). Only then does the interimtarget rate continue on its downhill traverse. The delay causes a newprogressive breath-rate target contour to be followed but, in theillustrative embodiment, with more or less the same slope.

Event 3 a in FIG. 3A is the first occurrence of the patient's breathrate not slowing down sufficiently to track the interim target. Thecontinuously decreasing interim rate target stops changing and remainsconstant. The pressure is now increased, as reflected in FIG. 3B. As thepressure is increased, the patient's breath rate decreases until itmatches the interim target, at which time the target resumes itsdownward slide. (Actually, as will become apparent from the flow chartof FIG. 10, a “match” occurs when the patient rate may still be a tinybit higher than the interim target rate. Being close enough issufficient reason to lower the interim target rate another increment.)Similar remarks apply to events 3 b and 3 c. Event 3 b also shows whathappens when the patient breath rate actually dips below the interimtarget, which will be discussed herein. In most cases, the targetdecreases until it is below the patient rate so that the breath pacing(increased pressure) can still further lower the patient rate.

The settling period referred to herein occurs (at least the first time)during sleep onset, where breathing is largely spontaneously triggered.The goal here is to offer comfortable, calming and/or minimal breathingassistance. The duration of the settling period can vary, for example,shorter for diurnal treatment sessions and longer for going to sleep.

During the adaptive rate phase the machine adapts to the patient's ownaverage rate. The difference between this and operation in thespontaneous mode may be small, and sometimes less important. Thedifference is that while in the spontaneous mode the machine follows thepatient's efforts in most instances regardless of the rate, in theadaptive rate mode there is a target rate which the trigger/cyclethresholds to be described herein aim to encourage. The PAP algorithmstarts from the base rate determined during the adaptive mode, and it isto this base rate that the machine reverts when the patient arouses, aswill be described.

FIG. 3B depicts how the pressure support (PS) variable increases as thebreath rate decreases. Exactly how the pressure changes is part of thealgorithm for decreasing the breath rate, and will be described below.The general idea is to increase the pressure slightly in order tolengthen the breath duration—since the increased pressure deliversgreater volume to the patient, the patient spontaneously slows down thebreathing. Those parts of the pressure curve that are flat reflectminimal feedback ‘error’—the breath rate is close enough to the interimtarget rate that there is often no need to increase the pressure.

FIG. 4 depicts a within-session time-course for breath pacing therapy,in which the optimal breath rate was achieved, but the patient'sbreathing rate then increased well above the optimal rate. In responseto the patient clearly preferring a significantly higher rate (event 4),pacing is aborted and the initial adaptive rate is restored, followingwhich the overall algorithm is executed once again. After stablebreathing is resumed and the settling time is satisfied, pacing isre-commenced (event 5). Basically, FIG. 4 reflects a recognition thatbreath pacing may fail, in which case the whole process starts overagain, using the previously determined adaptive rate. (It is, of course,possible to start over at the very beginning with event 1, although inmost instances it is not believed to be necessary.)

A conditioning time-course for the breath pacing therapy, showing howthe optimal breath rate may be achieved over many days/nights/weeks, isdepicted in FIG. 5. Optimal rate targets may be different for successivedays, and they get lower and lower. On each day, for the given slope,pacing takes place along a different path toward a different optimaltarget. Also illustrated is a change in the pacing algorithm's nominalassertiveness, where the deceleration in breath-rate is greater as thepatient becomes conditioned to slow breathing (i.e., the gradient of theprogressive pacing target gets steeper from day to day). If not fixed inthe machine, the progressive breath-rate target contour for each sessioncan be set by the patient or physician. Alternatively, the machine cankeep track of the overall course of therapy and adjust the targetcontours automatically (not shown in the flow chart of FIG. 10).

One of the primary mechanism that promotes slower breathing is a largertidal volume (air breathed per breath) delivered via controlled,progressive pressurization during inspiration, and controlledde-pressurization during expiration. This process is assisted byreluctant triggering and cycling that discourage a faster rate; easiertriggering and cycling of each breath phase is allowed only as thepatient approaches the interim target rate. FIGS. 6A and 6B illustratethe time course of flow and pressure for an individual breath duringinspiration early in the rate-lowering process.

The drawing shows the optimal Ti, the time from the start of inspirationat the left end of FIG. 6A to the end of inhalation when ideally theexpiratory phase of the cycle should begin. The patient has not yetprogressed to this level, however, and for the cycle depicted there is aprogressive or interim target Ti (see FIG. 6B) that is shorter than theoptimal Ti. For every target Ti there is a cycle threshold function orplot. The plot of FIG. 6A is for the particular interim Ti shown in thedrawing. It will be seen later in FIG. 7A that the cycle threshold plotfor a different Ti value is different.

Toward the end of inhalation, the flow will have decreased to the pointthat it crosses the cycle threshold at some time during the inspiratoryphase, at which point exhalation will begin. In the example shown, thecycle threshold is hit shortly before the target Ti is reached. Themachine switches to its exhalation mode when the flow is slightlynegative, as shown, i.e., very shortly after patient exhalation hasbegun. The pressure, at the usual end-expiratory level (EEP) at thestart of inspiration, did not quite rise to the target IPAP level, themaximum inspiratory pressure (minimum pressure plus current PS value).Since a spontaneous cycle has taken place with the patient starting toexhale slightly before the end of the desired Ti interval, the pressurenow follows the expiratory phase of the pressure profile. (see FIG. 6B.)(The dashed lines in FIG. 6B show what the pressure waveform would havebeen had the progressive Ti target been reached.)

The significance of the cycle threshold curve should be appreciated. Atthe start of the inhalation phase, there is a ‘reluctant’ cycling zone.The patient would need to oppose the inspiratory pressure for thethreshold to be reached and it is inhalation (positive flow) that isjust starting. But after some virtually guaranteed inspiratory machineoperation, it becomes easier and easier to cycle the machine to itsexhalation mode. Cycling occurs as the flow decreases, but the thresholdis crossed with higher flow later and later in a cycle. Thus thesensitivity increases, and it becomes easier to cycle (i.e., while thepositive flow is still significant) as the inspiratory phase lengthens.

When the inspiratory phase is relatively long, cycling is caused to takeplace while the patient is still inhaling, which is evident from FIG. 6Awhich shows the cycle threshold being positive as the length of theinhalation phase approaches the Optimal Ti. Traditionally, ventilatorsdo cycle (switch to exhalation mode) while flow is still inspiratory.That is because breathing against inspiratory pressure is uncomfortableand takes exertion. The cycle criterion is traditionally between 75% and25% of peak inspiratory flow. However, because the goal here is toprolong Ti beyond what is natural, the cycle threshold curveadvantageously requires a greater drop in inspiratory flow than is usualbefore cycling takes place.

It should be noted that if the flow does not cross the cycle thresholdcurve by some predetermined time interval after the Optimal Ti,mandatory cycling takes place. The machine switches to exhalationoperation.

FIGS. 7A and 7B are similar to FIGS. 6A and 6B, but here it is assumedthat the progressive Ti target has progressed all the way to the optimalTi. Now, because inhalation takes longer, the flow does not intersectthe cycle threshold curve until almost at the end of the desired Tiinterval. A spontaneous cycle occurs and the expiratory phase of thepressure template controls the applied pressure.

The important difference between FIGS. 6A and 7A is that the cyclethreshold curves are different. The cycle threshold curve that appliesduring any cycle depends on the interim rate target. (For each value ofthe target, there is a respective cycle threshold curve.) To encouragethe patient to become accustomed to a longer inspiratory phase, cyclingto the expiratory phase is made easier as the breath lengthens. Notethat the flow does not have to decrease as much in FIG. 7A as it does inFIG. 6A in order for the machine to cycle to its exhalation phase.

FIGS. 8A and 8B are similar to FIGS. 6A and 6B, but they show theexpiratory phase of a cycle, where triggering—the start of machineinspiratory operation—occurs early in the rate-lowering process when theprogressive Te target is well below the optimal Te. (The several figuresare not drawn to scale.) The pressure lowers from the starting IPAPvalue to the target EEP. The trigger threshold curve is very positive inthe early part of the expiratory phase, giving rise to a ‘reluctant’triggering region in which requires forceful inspiratory effort by thepatient for triggering to take place. As time progresses during theexpiratory phase, it becomes easier and easier to trigger as lesspositive flow is required to cause the machine to switch to itsinhalation mode of operation. (While a switch to exhalation mode takesplace while the patient is still inhaling, as discussed herein, a switchto inhalation mode occurs only after the patient has started to inhale,as is usual with conventional ventilators.)

FIGS. 9A and 9B show what happens toward the end of the rate-loweringprocess, where the patient's spontaneous expiratory phase is almost atthe optimal Te and the interim target Te is already there. Shown is aspontaneous trigger just short of the target Te. Comparing FIGS. 8A and9A, it should be noted that the flow does not have to increase as muchin FIG. 9A as it does in FIG. 8A in order for the machine to trigger toits inhalation phase. Should the optimal Te be achieved withoutspontaneous triggering, a machine trigger may be invoked. In general,the algorithm assumes a target Ti and a target Te at the start of abreath, but the target profiles may be truncated at a point if thepatient satisfies the trigger/cycle criteria.

Referring to the flow chart of FIG. 10, at the start of the machineprocess, the optimal Ti and Te are determined (typically, set by thephysician), the settling period (about 20 minutes) is initialized, andPAP (bi-level, but preferably following a template as shown in thedrawings) is applied. The actual current Ti and Te values aredetermined, as shown in step 50, along with the actual current breathperiod Ttot that is the reciprocal of the current respiratory ratederived by the analysis and conditioning block shown on FIG. 2B.

In step 52, three average values are calculated—the recent averagebreath period, Ttot-av, and the recent average inspiratory andexpiratory intervals, Ti-av and Te-av. The averages are calculated overan interval ranging from a few breaths upward. The three values aremoving averages, as is known in the art, the oldest sample values beingreplaced by the most recent sample values in the average calculations.These average values are the ones used in comparing the currentbreathing rate and its Ti and Te components to any instantaneous valueof interim target rate and its associated Ti and Te intervals.

Referring to FIG. 3A, it will be recalled that there is an interimbreath rate target that progressively decreases, but at any instant itis the current value of the interim target that is the input to theservo control. The current target is based on the time that has expiredsince the start of the progression as well as the progress of thepatient's breathing, as discussed herein in connection with FIG. 3A. Thecurrent breath rate target is also a function of the particular breathrate ‘curve’ (exemplified as straight lines in FIG. 3A) that is beingfollowed, a new desired progression being put in place whenever theactual breath rate is too high and the progressive rate needs adjustment(events 3 a, 3 b and 3 c in FIG. 3A). The breath rate ‘curve’ may alsobe a function of the particular day in the processing, as discussedherein in connection with FIG. 5. The physician can input the day fromwhich the controller can determine the starting slope, or the controllercan simply keep track of the treatment day and automatically determinethe starting slope.

At any given time the controller knows not only the interim breath ratetarget, but also both Ti and Te target components of it. The componentsare easily determined by providing a table or even a formula (e.g., afixed ratio) of Ti/Te pairs for each target breath rate. The algorithmcompares the actual current average Ti with the interim target Ti(tgtTi) and it compares the actual current average Te with the interimtarget Te (tgtTe). The average and interim target time intervals are thesame at the start of the processing, and the underlying pacing mechanismincreases the pressure gradually so that the time intervals can beincreased (i.e., the rate is lowered).

In certain embodiments, it is during the settling period that the targetvalues are set equal to the actual values. After the settling period,the actual breath pacing starts. In step 54, a test is performed todetermine if the settling period is over. If not, in step 56 the interimtarget breath rate is initialized again to equal the current averagebreath rate. This means is that the current average breath rate ismeasured and the interim target breath rate is set equal to it. And foran interim target breath rate, the two target values Ti and Te are set,as described herein.

Once the settling period is over, breath pacing begins. During eachiteration (iterations occur at intervals of from about 0.25 to 5minutes), the test of step 54 is answered in the affirmative. In step58, the two current average values of interest, Ti-av and Te-av, arecompared with respective targets tgtTi and tgtTe that are associatedwith the current target rate. Initially, during the first attempt atbreath pacing after the settling period, the average and target valueswill be the same since the targets are set to equal the averages in step56. (However, even though at the start of the pacing Ttot-av equalstgtTtot, the individual respective inspiratory and expiratory componentsmay differ.) But during subsequent passes the values will differ. If oneor both of the current Ti and Te averages differs appreciably from therespective interim target value tgtTi and tgtTe, then the situation isthat shown in FIG. 4—the patient is just not following the pacing andthe whole process has to start all over again.

The actual test is in step 60—is either current average interval muchtoo short compared with its respective target value? By too short ismeant that pacing is just not working and the current iterations shouldbe aborted. If either of Ti-av or Te-av is less than its respectivetarget by a predetermined threshold value, indicating patient oppositionto the current interim breathing rate, the whole process begins all overagain in step 62 with a new settling period. The previously determinedadaptive rate (see FIG. 3A) is used as the new interim target, and theprocess starts over again.

On the other hand, if the answer to the test of step 60 is in thenegative, it means that while the current inspiratory interval may begreater than the target Ti or the current expiratory interval may begreater than the target Te, the difference is small enough that theservo control may correct it. What we have is an event such as 3 a, 3 bor 3 c in FIG. 3A.

The ‘error’ in servo control terms is simply the difference between whatis desired (at the moment, this is the current interim target periodtgtTtot) and what is on hand (the current recent average breath periodTtot-av). The error is computed in step 64. In step 66 the pressure ischanged—the current PS target (the maximum template value) is increasedin accordance with a function H(E) that is dependent on the error. TheH(E) function can be implemented by a PID(proportional-integral-derivative) controller, although it could alsosimply provide a slight linear increase in the pressure in an attempt toextend the breath interval and reduce the error. The magnitude of apressure increment depends on how fast the iterations through the mainprocessing loop occur. The more iterations there are per minute, thegreater the pressure rise if in each iteration another increment isapplied. Typically, the increment magnitude and the rate of iterationsmight be such that the pressure rises at a rate of 1-3 cmH2O per minute.

It is possible that the error will be negative, i.e., the current recentaverage breath period is actually longer than the current interim targetperiod. In such a case, the H(E) function can be made equal to zero ifthe error is negative so that the pressure will not be changed until thetarget is increased sufficiently to make the error positive at whichtime a pressure increase will be called for. However, there are reasonsthat favor allowing the pressure to be decreased by a negative H(E) ifthe breathing rate is too slow and the breath period is actually longerthan the desired interim target. In certain embodiments, it may not bebest to employ a servo-regulation scheme that does not inherently allowthe pressure to fall because the overall ventilation of the patient isdetermined by both patient and ventilator together. So if the patientcontributes more effort at the desired rate, or if the ventilatory needsof the patient fall (e.g., transition to a different sleep state), andif the pressure support does not back off, this might possibly result indiscomfort and/or arousal. Also, once the optimal rate is achieved, itcould be that less pressure support is needed to maintain that rate thanwas needed to achieve it and for some patients the rate may be depressedtoo low (below the optimal rate) if the algorithm does not back off.While these patients should perhaps be contraindicated, the issue may beavoided, in certain instances, by allowing the servo control to decreasethe pressure as well as increase it.

The approach taken in certain illustrative embodiments is to lower thepressure if the error is negative. However, so that slight lowering ofthe pressure does not work much against the goal of increasing thebreath period, if the recent average rate is lower than the interimtarget rate (i.e., the error is negative), then the progressive ratetarget is adjusted to track the patient's recent average rate, that is,the algorithm adopts the slower of the two rates as the next target.Event 3 b in FIG. 3A is an example—the interim rate is shifted to thecurrent average rate (i.e., the interim total breath target is madeequal to the current average breath interval) because the current recentaverage rate has fallen below the interim rate. The progressive rateadjustment may not be immediate since it is the recent average rate thatis used and not the instantaneous rate, but this is often an acceptableprice to pay for allowing pressure changes in both directions.

The interim rate adjustment just described occurs in step 72, which willbe described herein. Prior to that, in step 66, the pressure is adjustedup or down as described herein. Then, in step 68, still another test isperformed. The error is compared with a threshold value denominated asthe hysteresis error. For the moment consider that the threshold valueis zero. If the current rate is too high, it means that the error ispositive. Since the error is greater than the threshold value, theanswer to the test of step 68 is yes. No adjustment is made to theinterim target because the breath period is still too short. Thepressure will be increased (as a result of step 66) and perhaps now thebreath period will lengthen to the period of the interim target. Theprocessing returns to step 50 for another iteration.

If the answer to the test of step 68 is no, it means that things aremoving along nicely and the breath period has lengthened to the interimtarget. Starting in step 70 the various adjustments discussed earlierare made.

Before considering these adjustments, however, it should be noted thatthe hysteresis error value may be positive instead of zero. This simplymeans that the answer to the test of step 68 is no even if there is aslight positive error, less than the hysteresis value. The recentaverage breath period may be a bit too short compared with the currentinterim value, but it is still treated as having satisfied the interimtarget so that target values and parameters are adjusted. Hysteresisallows the algorithm to tolerate variation in rate below the interimtarget without sustained decrement in pressure support. The pressuresupport will reduce, but perhaps for only one iteration or a fewiterations of the loop since a new (lower) target will be adopted.

The test in step 70 checks whether overall success has been achieved. Ifthe interim rate target now in effect is less than the optimal ratetarget, then the goal has been reached, no changes are necessary and anew iteration begins. But if the interim rate target has not beenadjusted all the way to the optimal value, it means that the breathingrate is still faster than optimal, and starting in step 72 adjustmentsare made in order to slow down the breathing still further. The firstadjustment that is made is that described herein—a new interim targetperiod (tgtTtot) is selected. It is the lower of two values. One valueis the next value in the progressive target (see FIG. 3A). The othervalue is the recent average breath period. The longer of the two timeinterval values is selected. The reason that the recent value may trumpthe progressive target is that if things are going well, and thepatient's breath is slowing down even faster than expected, advantagemight as well be taken of the fact.

After the new interim target is selected, it is necessary to set its twoindividual components, tgtTi and tgtTe since the individual values areneeded in step 58. This is done in step 74. As mentioned herein, the twovalues can be taken from a table or a formula can be used to derivethem.

The pressure versus time profile is fixed in the certain illustrativeembodiments, but the amplitude and time axes are scaled in step 76 inaccordance with the new maximum pressure target PS determined in step 66and the new interim target period tgtTtot determined in step 72. (Seepressure profile at output of turbine 26 on FIG. 2B.)

Referring to FIGS. 6A and 7A, it will be recalled that the cyclethreshold plot changes with the progressive Ti target, it being easierto switch to cycling as the target Ti lengthens (i.e., the machineswitches to the exhalation mode when the flow intersects the cyclethreshold plot at higher values). Using the value of tgtTi determined instep 76, in step 78 the new cycle threshold plot is determined.Similarly, and with reference to FIGS. 8A and 9A and the descriptionherein of how the trigger threshold plot changes with the target Te, instep 80 the value of tgtTe determined in step 76 is used to select thenew trigger threshold plot. The processing then continues with a newiteration starting with step 50.

It is inherent in the algorithm or flow chart of FIG. 10 thatspontaneous breathing is retained. Once the optimal rate is achieved,pressure support may diminish. Therefore, should the patient beventilated below his/her apneic threshold during sleep, thetriggering/cycling encourage Ti and Te to be maintained at the optimalrate, and pressure support diminished. This will in turn reduceventilation and ensure the return of spontaneous breathing.

Although the inventions have been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the inventions.Thus it is to be understood that numerous modifications may be made inthe illustrative embodiments of the inventions and other arrangementsmay be devised without departing from the spirit and scope of theinventions.

What is claimed is:
 1. A ventilator for slowing a patient's breathingduring sleep by using positive pressure therapy, the ventilatorcomprising: a blower for applying a variable pressure waveform to thepatient's airway, at least one sensor for detecting the breathing rateof the patient, and a controller, wherein the controller is configuredto: i) set a current interim breathing rate target, ii) cause the blowerto periodically increase the magnitude of the pressure waveform when thepatient's breathing rate is greater than the interim breathing ratetarget in order to lengthen the patient's breath duration, and iii)periodically reduce the interim breathing rate target in response todetection of the patient's breathing rate slowing down toward thecurrent interim breathing rate target; wherein the controller causes thevariable pressure waveform to cycle from an inhalation phase to anexhalation phase when the patient airflow decreases to a cyclethreshold, the cycle threshold being a function of flow versus timewithin a breath and generally increasing with time; and whereindifferent interim breathing rate targets have different cycle thresholdfunctions, and the cycle threshold functions allow easier cycling as theinterim breathing rate targets decrease.
 2. The ventilator of claim 1wherein the controller causes the variable pressure waveform to triggerfrom an exhalation phase to an inhalation phase when the patient airflowincreases to a trigger threshold, the trigger threshold being a functionof flow versus time within a breath and generally decreasing with time.3. The ventilator of claim 2 wherein different interim breathing ratetargets have different trigger threshold functions, and the triggerthreshold functions allow easier triggering as the interim breathingrate targets decrease.
 4. The ventilator of claim 1 wherein thecontroller causes the magnitude of the pressure waveform to decrease ifthe patient's breathing rate is less than the interim breathing ratetarget.
 5. The ventilator of claim 4 wherein the controller causes themagnitude of a pressure increase or decrease to be a function of thedifference between the interim breathing rate target and the patient'sbreathing rate.
 6. A ventilator in accordance with claim 1 wherein thecontroller causes the interim breathing rate target to be reduced alonga predetermined path, but causes the reduction to pause if the patient'sbreathing rate is excessively high for the current interim breathingrate target.
 7. A ventilator in accordance with claim 1 wherein thecontroller causes the duration of the pressure waveform to be adjustedin accordance with the current interim breathing rate target.
 8. Aventilator for slowing a patient's breathing during sleep by usingpositive pressure therapy, the ventilator comprising: a blower forapplying a variable pressure waveform to the patient's airway, at leastone sensor for detecting the breathing rate of the patient, and acontroller, wherein the controller is configured to: i) set an interimbreathing rate target, ii) cause the blower to increase the magnitude ofa variable pressure waveform that is scaled to the interim breathingrate target if the patient's breathing rate is greater than the interimbreathing rate target in order to lengthen the patient's breathduration, the magnitude of the pressure increase being a function of thedifference between the interim breathing rate target and the patient'sbreathing rate, and iii) reduce the interim breathing rate target inresponse to the patient's breathing rate slowing down toward the interimbreathing rate target wherein said controller causes the variablepressure waveform to cycle from an inhalation phase to an exhalationphase when the patient airflow decreases to a cycle threshold, the cyclethreshold being a function of flow versus time within a breath andgenerally increasing with time; and wherein different interim breathingrate targets have different cycle threshold functions, and the cyclethreshold functions allow easier cycling as the interim breathing ratetargets decrease.
 9. The ventilator of claim 8 wherein said controllercauses the variable pressure waveform to trigger from an exhalationphase to an inhalation phase when the patient airflow increases to atrigger threshold, the trigger threshold being a function of flow versustime within a breath and generally decreasing with time.
 10. Theventilator of claim 9 wherein different interim breathing rate targetshave different trigger threshold functions, and the trigger thresholdfunctions allow easier triggering as the interim breathing rate targetsdecrease.
 11. A ventilator for slowing a patient's breathing duringsleep, the ventilator comprising: a blower adapted to be coupled to apatient's airway, at least one sensor for monitoring the breathing ofthe patient, and a controller, wherein the controller is configured to:i) set an interim breathing rate target, ii) cause the air supplied tothe patient's airway from the blower during patient breaths to increaseif the patient's breathing rate is greater than the interim breathingrate target in order to lengthen the patient's breath duration, iii)reduce the interim breathing rate target in response to the patient'sbreathing rate slowing down toward the interim breathing rate target,and iv) interrupt air increase and interim breathing rate targetreduction if the patient exhibits opposition to breath durationlengthening, wherein the air increase and interim breathing rate targetreduction to take place over a period of minutes to hours in the absenceof patient opposition; wherein the controller causes the blower to cyclefrom an inhalation phase to an exhalation phase when patient airflowdecreases to a cycle threshold, the cycle threshold being a function offlow versus time within a breath and generally increasing with time; andwherein different interim breathing rate targets have different cyclethreshold functions, and the cycle threshold functions allow easiercycling as the interim breathing rate targets decrease.
 12. Theventilator of claim 11 wherein the magnitude of an air increase is afunction of the difference between the interim breathing rate target andthe patient's breathing rate.
 13. The ventilator of claim 11 wherein thecontroller causes the blower to operate so as to synchronize it to thepatient's breathing rate rather than the interim breathing rate target.14. The ventilator of claim 11 wherein the controller causes the blowerto trigger from an exhalation phase to an inhalation phase when patientairflow increases to a trigger threshold, the trigger threshold being afunction of flow versus time within a breath and generally decreasingwith time.
 15. The ventilator of claim 14 wherein different interimbreathing rate targets have different trigger threshold functions, andthe trigger threshold functions allow easier triggering as the interimbreathing rate targets decrease.
 16. The ventilator of claim 15 whereinthe controller causes the magnitude of an air increase to be a functionof the difference between the interim breathing rate target and thepatient's breathing rate.
 17. The ventilator of claim 16 wherein thecontroller causes the blower to operate so as to synchronize to thepatient's breathing rate rather than the interim breathing rate target.18. The ventilator of claim 15 wherein the controller causes the blowerto operate so as to synchronize to the patient's breathing rate ratherthan the interim breathing rate target.
 19. The ventilator of claim 14wherein the controller causes the magnitude of an air increase to be afunction of the difference between the interim breathing rate target andthe patient's breathing rate.
 20. The ventilator of claim 14 wherein thecontroller causes the blower to operate so as to synchronize to thepatient's breathing rate rather than the interim breathing rate target.